The present invention relates to a detecting apparatus used in a vehicle, such as an automobile, for detecting a steering state of a steering member thereof.
For example, in an automobile, generally, a steering assisting apparatus of an electric type or a hydraulic type is connected to a steering system between a steering member and a steered wheel and a steering load of a driver is alleviated by applying steering assisting force from the steering assisting apparatus to the steering system. The steering system includes a steering shaft having an input shaft connected to the steering member and an output shaft connected to the steered wheel by interposing a torsion bar between the input shaft and the output shaft and a direction of the steered wheel is changed in accordance with driver's operation for steering the steering member.
Further, in the above-described automobile, it is requested to prevent steering feeling of the driver from being deteriorated by exerting a pertinent steering assisting force in accordance with the steering operation and there is provided a steering state detecting apparatus for detecting a steering state at the steering member, specifically, steering torque applied to the member, steering angle or the like.
A general steering state detecting apparatus is provide with two sensor portions for respectively outputting output signals in accordance with rotation of the input shaft and the output shaft and the steering torque applied to the steering member is calculated by calculating a torsional angle of the torsion bar by acquiring a relative angular displacement between the input and the output shafts based on a signal phase difference of the sensor portions. Further, the sensor portions each is provided with a target in a shape of a circular ring having a teeth portion comprising projected streaks made of a magnetic body inclined to an axial direction of a corresponding one of the input and the output shafts in the same direction by an equal angle and a magnetic sensor comprising a magnetoresistive element arranged to be opposed to the teeth portion and the magnetic sensor outputs an output signal periodically changed in accordance with rotation of the corresponding one of the input and the output shafts (for example, refer to Patent Literature 1).
However, according to the above steering state detecting apparatus, the magnetic sensor detects a change in a distance to the teeth portion inclined to the axial direction by the equal angle in accordance with rotation of the corresponding one of the input and the output shafts based on a change in an intensity of a magnetic field and outputs the periodically changed output and therefore, it is necessary to attach the sensor and the target with high accuracy since an attaching allowable error between the magnetic sensor and the teeth portion is small in the axial direction and a long period of time is required for integrating operation thereof.
In order to solve the above-described problem, the applicant provides a detecting apparatus capable of increasing an attaching allowable error between a teeth portion and a magnetic sensor in an axial direction in comparison with that of the above apparatus by using a target in a shape of a spur gear having a teeth portion projected substantially at equal intervals in a peripheral direction in place of the target (for example, refer to Patent Literature 2)    Patent Literature 1            JP-A-2002-29431            Patent Literature 2            JP-A-2002-116095        
However, according to the above steering state detecting apparatus described in Patent Literature 2, when the torsion bar is twisted by a large amount, depending on a number of teeth of the target teeth portion, output signals (rotation detecting waveforms) of input and output shafts outputted by a corresponding one of magnetic sensors may be shifted from each other by an amount of one period or more. Therefore, the relative angular displacement of the input and the output shafts cannot instantly be detected by using the rotation detecting waveforms from the respective magnetic sensors and it is necessary to carry out a determining process of whether the rotation detecting waveforms are shifted from each other by an amount of one period or more by counting numbers of waveforms (periods) of the respective rotation detecting waveforms of the input and the output shafts from a time point of starting (detecting) the steering operation of the driver. As a result, the steering state immediately after inputting power source (starting automobile) may not be able to be detected or an increase in a processing of the detecting apparatus or large-sized formation of a circuit scale may be brought about.